1. Technical Field
The present invention relates to an image analysis apparatus and method for analyzing a three-dimensional distribution of ventilation volume of lungs using three-dimensional images representing motion of the lungs during respiration, and a program for causing a computer to perform the image analysis method.
2. Background Art
Recently, high quality three-dimensional images are used in image diagnosis with the advancement of medical equipment (e.g., multi-detector CT, hereinafter referred to as multi-slice CT, and the like). In the multi-slice CT, a plurality of rows of detectors for detecting X-ray is provided and a plurality of tomographid images can be obtained at a time by one rotation of the rows of detectors around a human body. The number of detectors has been increased exponentially by the technological advancement, and recently a multi-slice CT with a maximum of 320 rows has been used. The time required for the detectors to make one rotation around a human body is about 0.35 seconds and the imaging range in an axis direction of the human body extends over to 16 cm. Consequently, in the regions, such as head, heart, and the like, it is possible to obtain a plurality of three-dimensional images in time series at a short time interval. By displaying, in time series order, an organ of interest included in the three-dimensional images obtained in time series as described above, it is possible to observe the motion of an organ, such as a heart or the like, like seeing a moving picture. It is also possible to observe, in the head and the heart, how a contrast agent or the like circulates through the body in the three-dimensional images.
In the meantime, the amount of radiation received by the subject is increased with the increase in the imaging range and the number of imaging times, but developments of image reconstruction algorithms that allow high quality images to be obtained with a less amount of radiation have been conducted in manufactures of modalities, such as imaging equipment. Consequently, imaging over a wide range with a prolonged time, which required a large amount of radiation in the past, has also been conducted.
With respect also to lungs, it is becoming possible by such advancement in modalities to obtain three-dimensional images of the entire lung region (about 30 cm) in time series at a short time interval during a series of respiration cycle of inhaling largely and exhaling. This allows the motion of the lungs due to respiration to be observed like seeing a moving image.
In the meantime, bronchial asthma and lung emphysema may be cited as respiratory diseases. Heretofore, with respect to these diseases, spirometers (spirometry) or nuclear medicine studies, such as lung scintigraphy of SPECT, have been used to check the ventilation volume of the lungs. Here, the spirometry measures the volume of breath exhaled by the subject and the exhaling time, and is recommended as tests for diagnosing lung diseases such as COPD (Chronic Obstructive Pulmonary Disease) and the like. The spirometry may obtain a ventilation volume of the entire lungs and the ventilation curve (vital capacity curve), but is unable to check which portion of the lungs has a reduced ventilation volume. The SPECT scan may visualize the state of ventilation with respect to each region, but is unable to obtain detailed information due to relatively poor image quality. Further, it has a problem that the amount of radiation received by the patient is increased as it is necessary to inject a radioactive material into the body of the patient.
Consequently, a method that extracts lung regions from CT images of two phases of exhalation and inhalation, aligns the extracted lung regions using a non-rigid registration method and obtains a displacement vector field in the lung region, and calculates a local ventilation volume of the lungs by calculating a divergence of the displacement vector field is proposed (refer to Japanese Unexamined Patent Publication No 2005-028121).
Further, a method that disposes an air passage in each of three-dimensional time series images representing lung motion, extracts a three-dimensional grid point between air passages in each three-dimensional image, and measures a spread of tracheomalacia by measuring the volume of the extracted point is proposed (refer to U.S. Patent Application Publication No. 20080009683).